Ferromagnetic microwave device



July 7, 1959 EN 2,894,224

FERROMAGNETIC MICROWAVE DEVICE Original Filed May 19, 1954 I INVENTOR.

Arthur H. lversen,

' AGENT.

CalISGqCOHSiClBTablG heating of the compact.

United States Patent 2,894,224 FERROMAGNETIC MICROWAVE DEVICE Arthur H.Iversen, Santa Monica, Calif., 'assignor to Hughes Aircraft Company,Culver City, Calif., a corporation of Delaware Original application May19, 1954, sen No. 430,341. Divided and this application December 12,1'956, Serial No. 627,930

2 Claims. (Cl. 33334) This invention relates to hermetically-sealedferromagnetic dielectric materials, and more particularly tovitreous-coated ferromagnetic dielectric compacts.

This is a divisional application of application Serial Number 430,841,filed May 19, 1954.

Ferromagnetic dielectric materials, known in the art simply as ferrites,are crystalline ceramics in which the major constituent is iron oxide.These ceramics are generally formed into a compact by aclosely-controlled sintering process from a powder mixture whichincludes a plurality of bivalent-metal oxides. Ferrite compacts have awide range of physical and electrical properties which arede'penden't ontheir composition and the sintering technique employed in theirformation.

:The ferromagnetic dielectrics are particularly useful in microwaveapplications because at these frequencies they exhibit a characteristicknown as the Faraday effect.

However, in use, the heating effect, power factor, andpolarizationgrotating ability of the ferrite compacts are adverselyaifected by increasing moisture content.

An example of such utilizations of the ferrite compacts is shown incopending application, Serial No. l 19,259 by Willard A. Hughes, filedon March 29 1954. In this example, the ferrite compact is utilized torotate incident wave energy 45 degrees for transmission to a load. Thereflected energy is rotated another 45 degrees in again traversing theferrite compact so that it has a final pclarization at the source end ofthe ferrite which is normal to that of. the incident energy. Thisrelationship between the planes of polarization of the incident andreflected energy makes it feasible to divert the reflected energy out ofthe working system and so prevent it from traveling 'ba'ck to thesource.

In addition to the utilization of the ferrites in microwave transmissionsystems, they are also useful in certain types of electron dischargedevices, such as, for example, the traveling-wave type. Thesusceptibility of the ierrites to release occluded gases over a longperiod of time is an impediment to this type of use because of thenecessary high vacuum which must be maintained in the electron dischargedevice. it follows that this problem can be. obviated by hermeticallysealing the ferrite cornpact in accordance with the present invention.Accordingly, it follows that if the ferrite compacts can be hermeticallysealed. in an envelope or skin in a dry condition,,their usefulness willbe very much enhanced. The

manner in which the ferrites are utilized, as Well as their inherentphysical properties, interposes a number of difficulties in the way ofachieving a satisfactory hermetic envelope.

The ferromagnetic dielectric compact or ferrite is utilized in highintensity microwave applications which it follows that. ascalingskin forthe compact must be capable of withstanding a temperature of at leastseveral hundreddegrecs centigradewithout deterioration, cracking orotherwise developing porosity. The substances capable of perice 2forming a scaling function at these temperatures are the vitreousproducts, notably glass.

An obstacle in the way of utilizing a vitreous skin on ferrite compactsis the inherent susceptibility of the compact to deterioration, 'i.e.,cracking, due to thermal shock. That is, the tensile and compressivestrengths of the ferrite compact are insufficient to prevent disruptionof the compact due to sudden heating to a high temperature of the outersurfaces thereof. Consequently, since a glass surface must be applied tothe compact at the molten temperature of the glass, it is necessary toutilize a glass which has a temperature of melting which is not so highas to cause breakup or cracking of the compact. On the other hand,because, as previously mentioned, the ferrite compact can become quitehot in operation, it is necessary to utilize a glass which has a highenough melting temperature to preclude deterioration of the glass skinor envelope.

In addition to the above problems involved in providing a ferritecompact with a vitreous coat or skin, it is also necessary that thecompact with its vitreous skin withstand variations in temperature overseveral hundred degrees Centigrade without deterioration of either thecompact or seal. This requires that the vitreous substance utilized forsealing the compact have a coeflicient of thermal expansion suflicientlysimilar to that of the ferrite compact to limit the stresses in eitherthe skin or the compact itself so as to avoid deterioration of eitherthe skin or the compact.

It follows that a prime objective of this invention is to provide asealing coat or skin onto a ferromagnetic dielectric compact. v i

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will become apparent fromthe following description considered in connection. with the acoompanying drawing, made a part of this specification.

In the drawing:

Figure 1 is a partial broken section of a hermetically sealedferromagnetic ferrite device of the present invention; and V Fig. 2 isan enlarged cross-sectional view, section 2-2, of the device shown inFig. 1.

Referring to the figures of the drawing, a typical embodiment of thepresent invention is shown comprising an elongated figure of revolutionferrite compact or slug 10 having a glass coating or skin 11 whichhermetically seals the compact 11 from the surrounding medium. In theembodiment shown, the diameter of the middle portion of the compact isof the order of from 0.1 to 1.0 inch and its length is generally withinthe range of from one to four inches, the dimensions being determined bythe particular application. Inasmuch as ferrites may be used in numerousother forms, such as, for example, hollow or solid cylinder, it isreadily apparent that the scope of the present invention is not limitedto the sizes mentioned or particular configuration shown.

In order to provide a suitable glass skin on the ferrite compacts, it isfirst necessary to determine the physical properties of the ferrite.Ferrites of many varied and diverse compositions are commerciallyavailable. Many different compositions are employed because of theunusually large number of combinations of different useno physicalproperties exhibited by all of them. Representative samples of aferromagnetic ferrite may contain the, following constituents: MgO, MnOCuCO ZnO, Z (unstable), 1350, C210, Zr, ZrO The ferrite are normallychemically reactive with lead peroxides and bora'tes, but they are notreactive with oxides of aluminum or magnesium or the carbonates ofsodium or calcium.

terial and stick thereto.

tivity and high permeability in comparison to powdered iron materials.Their specific gravity lies between 4 and 5 and their dielectricconstant is about 9. These and other peculiar properties have led to awide field of applications, many of which, as previously mentioned,require ferrites to be hermetically sealed.

The thermal coeflicient of expansion of ferrites is primarily limited tothe range 71 to 93X inch per inch per C. They have a relatively highmodulus of elasticity and their ultimate strength is usually about10,000 p.s.i., i.e., pounds per square inch, in compression and 2000p.s.i. in tension. are generally the most important when correlated withthose of glass, the thermal expansion coefficient normally being themost important of the three.

To produce a hermetic seal that will withstand high vacuum and largetemperature variations, glass probably has better physical and chemicalproperties than any other material.

In obtaining a glass seal for a material it is necessary for the glassto wet and to match the material. Since most all glasses wet metaloxides, there is obviously no problem presented with respect to thisrequirement in coating ferrites since they are almost totallyconstituted of the oxides of bivalent metals. What .is normally meant bymatching a glass to a material is to seek out a glass having a thermalcoeflicient of expansion near enough to that of the material to preventa structural failure in either by stresses set up by differentialexpansion.

In the manufacture of a good seal, being able to match a glass and amaterial from the standpoint of thermal expansion, i.e., being able tokeep the strains in the glass or the material below their breakingstrength in case their thermal expansions donot match, is not sufiicientwhen accomplished only at room temperature because the glass andmaterial must first of all be heated to a high temperature in order tomake the seal. For this purpose, the glass is heated to its workingtemperature, the temperature above that at which it begins to soften orchange its shape. In fact, the glass should be rendered plastic orfluid-like so that it will wet the ma- Such a temperature is in theneighborhood of 800 C. or higher, although, as it will be seen, one typeof useful glass has a working temperature of 560 C.

The working temperature of a glass is to be distinguished from thesoftening temperature which may be as low as 440 C. and which is definedas being the temperature at which the glass becomes suliicientlyyielding .so that strains are relieved at an extremely rapid rate orwithin a very short period, for example, less than one minute. The sealis then allowed to cool directly to room temperature or, as is the moreusual case, the cooling process is temoprarily arrested at apredetermined elevated temperature, termed the annealing point or thelowest temperature below the softening temperature at which 90% of theinternal stresses of the glass will be removed in about fifteen minutes.

In choosing a suitable glass in which to seal a ferrite, it is to benoted that a considerable choice as to which and how many raw materials,in addition to sand, are to be incorporated into a batch of a usefulsilicate glass. Silica glass or fused quartz and fused silica, made froma batch consisting of sand alone, appears to be the ideal in glass innearly all its physical and chemical properties. However, if too high apercentage of sand is utilized, the glass formed in a batch involvesmelting temperatures of about 1700 C. which are too expensive andimpractical even with the best commercial glass-furnace These threephysical propertiesefiicient of expansion, being about 8 l0-"-two-component silicate glasses, which are unsuitablefor making glassarticles because of water solubility or de- ;vitrificationi.e.,crystallization, are classified in five chemical types: soda-limeglasses, sometimes called simply soda glasses and sometimes limeglasses; .lead glasses; and three new low-expansion and thereforeheat-resistant type glasses, namely, borosilicate glass; and 96% silicaglass, and pure silica glass or quartz. This listing is .more or less inorder of increasing silica content, in-

creasing mechanical durability, improving electrical properties,increasing cost, increasing shaping difficulty, and decreasing thermalexpansion coefficient.

The composition of these types, given in Table I, are the analyses ofthe finished glasses for elements, except oxygen, converted to oxideequivalent. a

Table l.-C0mpositions of commercial glasses Composition, Percent aComponent Soda-lime Lead Borosilicate 96% Silica Silica glass SlOz-70-76 (72) 53-68 (68) 73-82 96 19.8 N820 1218 (15) 510 0) 3-10 (4) K@O0-1 1-10 (6 0-4-1 OaO- 5-14 (9) 0-6 (1) 01 PbO- 15-40 (15) 0-10 B103-6-20 (14) 3 A1 0; 52.6 (1) 0 2 2-3 (2) MgO 0-4 (3) a The figures inparentheses give the approximate composition 0! a typical member.

In order to make a successful seal, the expansion of ferrite 10 andglass 11 must be substantially theflsa me over the temperature rangewithin which the glass is elastic. A large difference in expansionbetween them produces stresses which may cause either or both to crackwhen cooled to room temperature. Some degree of differential expansion,however, is tolerable and sometimes desirable. The relaxationcharacteristics of the glass determine the upper temperature limit towhich matched 'glasses and the lead glasses. The thermal coeflicientsofeach of these types have the following respective ranges: 67 to 105 and9 to 10-' inch per inch per C. Although soft glasses may go lower topossibly 67x10 inch per inch per C., few will go as high as 140x10 inchper inch per C. Fused silica or 100% silica glass has a coefiicient ofexpansion which is too low for the present purposes, being about 5.5)(10inch per inch per C. and 96% silica glass likewise has too low a co-.inch per inch per C. The glasses called borosilicate glasses contain 5%or more of boric oxide. They are also usually too hard to seal aferrite, the range of their thermal expansion coeificients being about13 to 60Xl0' inch per inch per C.

Glass in general has an ultimate strength in compresp.s.i., except forvery small glass fibers.

T It is, however, pertinent to note that the use of any glass with aferrite QQQQQQQ demands that he thermal rnansionroeflicient .be welmatched to prevent a struc ural f ilure Particular y 111 the ferritebecause of the low intimate istltength of the latter. e

Next in importance to therthermal expansion coeificient is the modulusofelasticity of a glass. .is :tme .because the glass should also be.pliable at low temperatures to be able totake alarge deformation withvery little stress, i.e., its modulus of .elasticity should be as low1as possible. Youngs modulus does not appear to be especiallyrelatedtothe fhardness of a glass or to any particular element of itscomposition. It is also true that there appears to be no correlationbetween Youngs modulus and the thermal expansion .coeflicient. Thismodulus of elasticity, however, ,does not normally vary appreciably forsoft glasscstofcommon compositions and is, therefore, not nearly .soimportantas the thermal expansion of glass. For example,.,all Youngs:moduli of glasses fall within the range 65 to 127 x p.s.i. Some s as was 6 1 0 PB -i withta therma expans efficient of 796x10 inch per inch C.The moduli of hard glasses are scattered over an equally wide range,going as low as 68 l0--' p.s.i. for a glass having a thermal expansioncoeflicient of 32x10 inchper inch per C. andgoing as high as 127 ,10;p.s.i. fortthermal expansion coeflicient of 42 l0' inch per inch per C.

Although the soft \glasses are generally satisfactory for coating aferrite, there are a few glasses which may be used to particularadvantage to reduce the risk of structural failures during themanufacture of the seal and especially during the handling of thecomponent materials. For example, in the case of ferrites having athermal expansion coefficient falling within the range 71 86 l0" inchper inch per C., they have an effective expansivity between 65 and l00l0 inch per inch per C. A glass having characteristics approximatingthese is commonly known as lime glass. This glass is composed of 73.3%SiO 15.6% Na O; 5.4% CaO; 3.8% MgO; 1.4% R 0 0.5% K 0 and has aneffective expansivity of 92i2 10-' inch per inch per C., and a Youngsmodulus of 98 10"' p.s.i.

It has also been found that a seal between the soft glasses known to thetrade as 0080 and 7570 manufactured by the Corning Glass Company may beused in hermetically sealing the ferrites of the above description. TheCorning Glass Company designates clear sealing bulb glass and solderglass with the number 0080 and 7570, respectively. These glasses arekept as closely as possible to a standard chemical composition at alltimes.

In order to apply the glass coating 11 to the ferrite slug 10, asuitable glass of the above-described types is first powdered and thensuspended in a liquid, such as, for example, water, methyl alcohol, or asolution of nitrocellulose in amyl acetate. This suspension of powderedglass is then brushed or sprayed uniformly over the ferrite slug 10 oralternatively the ferrite slug 10 may be immersed in a suspension ofpowdered glass in the solution of nitrocellulose in amyl acetate.

The ferrite slug 10 covered with the suspension of powdered glass isthen heated in a controlled atmosphere to the working temperature of theglass and then slowly cooled. This process is repeated until the glasscoating 11 has a glaze finish and is of a suitable thickness. Thethickness of glass coating 11, depending on the uses to which theferrite is to be put, is of the order of from 0.003 inch to more than0.06 inch, the thinner coatings being used where permissible in that theelectrical losses are less.

Referring again to Fig. 1, it is seen that glass coating 11 of thedevice has elongated pointed ends 12 where the glass is considerablythicker than the remaining portions of the coating. The device, aspreviously mentioned, is employed to rotate the plane of polarization ofthe electric field of a propagated wave. In order to accomplish this, itis necessary for the electromagnetic wave to propagate through themedium .lQf will? de ice." in that the dielectric constant of ferritequite h gh. ther i ge erally .aimpedanee mismatch h 8 111smediumeurrounding the device and the mcdinmrofrthe f it slug .10. Thus,if the :glass :has adielectricrconstant approximately equal tothegeometric mean of tthfi dielectric constants of the surrounding andferrite me ms. t may e empl ye t substant al y improve the i d e matchof the ferrite lug :10 to 111.653.1 3.001 ingrruedium. The thickness ofglass points 12 for optimum matching as .measured along the longitudinalaxis of the device is of the order of one-half guide wavelength.

A glass suitable for making the glass points 12-isknowu as Corning glass0120. This glass hasathe particularly low dielectric constant of 6.6which is necessary to approximate the aforementioned geometric mean ofthe dielectric constants of the surrounding medium and the ferrite. TheCorning:0l20 glassis a-clear potash :soda lea glasshaving a thermalexpansion of '89 910 tinch per finch per 2G,, and :a Youngs modulus ofx10 p sgi. which suitable for .use in conjunction with the glassconstituting'therglass coating 11.

it has "been found .inflatternpting to hermetically seal sorne ferritesthat ,they are sufliciently porous so ,as .to continuously absorb theglass when heated to the fluid state. This effect is to be avoided inthat the characteristics of the ferrite are deleteriously afiected.Accord ing to the present invention, ferrites of this type are sealed byfirst applying a coating of glass 13 having a high working temperatureto the ferrite. This coating will generally not effectively seal theferrite, but it will prevent subsequent coatings of glass having a lowerworking temperature from being absorbed into the ferrite.

More particularly, this process of hermetically sealing a porous ferriteis as follows:

(1) Suspend powdered Corning glass 0080 in water.

(2) Brush glass 0080 onto the ferrite.

(3) Heat glass 0080 in an oven in air to its sintering temperature,i.e., the temperature at which substantial fusion but not fluidity isindicated, e.g., 700-800 C.

(4) Allow the glass and the ferrite to cool.

(5) Brush a liquid suspension of powdered Corning glass 7570 over thesintered 0080 glass.

(6) Heat the ferrite and glasses to the working temperature of the 7570glass, i.e., about 560 C.

(7) Allow both glasses and ferrite to cool.

Corning glass 7570 by itself has utility in the manu' facture of theferrite seal of the present invention in that its working temperature is560 0, whereas most other glasses have a working temperature above 800C. The use of this glass reduces the risk of structural failure due tothe low thermal shock resistance of fcrrites. Cornlng glasses 7570 and0080 have thermal expansion coetficrents of 84 and 92 l0 inch per inchper C., respectively, and in this regard are useful in preventingdllferential stresses. Youngs modulus of Corning 7570 is w1th1n theusual soft glass range. Likewise, Corning 0080 has a Youngs modulus of98x10 p.s.i.

A few other glasses may also be used to hermetically seal a ferritematerial. Among these glasses are clear seallng glass," which isdesignated by the present Corn- Ingnumber code, by 8870. Its applicationis somewhat llmited, particularly because of its high dielectric constant, viz., 9.5; however, its thermal expansion coefficient is 91x10inch per inch per C. and a large advantage accompanying its employmentis its modulus of elasticrty, 76 10 p.s.i., which is comparatively low.

What is claimed is:

1. A device for rotating the plane of polarization of an electromagneticwave comprising: an elongated figure of revolution ferrite compact slughaving a cylindiical mid-portion and tapered end portions, said compactslug being composed of a ferromagnetic ferrite material havingpredetermined moisture sensitive and heat sensitive physicalcharacteristics; and a multi-layer protective glass coating disposedover the entire exposed surface of said slug for hermetically sealingsaid slug, said coating comprising a first relatively very thin layer ofa glass having a predetermined working temperature which is lower thantemperatures harmful to said ferrite material, said first layersubstantially sealing said slug while not appreciably soaking into saidslug, and a second layer of glass fused over said first layer and havinga working temperature which is lower than said predetermined workingtemperature, said second layer totally sealing said first layer andthereby hermetically sealing said ferrite slug without appreciablesoaking of said second layer of glass into said ferrite slug.

2. A device for rotating the plane of polarization of an electromagneticwave comprising: an elongated figure of revolution ferrite compact slughaving a cylindrical mid-portion and tapered end portions, said compactslug being composed of a ferromagnetic ferrite material havingpredetermined moisture sensitive and heat sensitive physicalcharacteristics; and a multi-layer protective glass coating disposedover the entire exposed surface of said slug for hermetically sealingsaid slug, said coating comprising a first relatively very thin layer ofa glass having a predetermined working temperature which is lower thantemperatures harmful to said ferrite material, said first layersubstantially sealing said slug while not appreciably soaking into saidslug, and a second layer of glass fused over said first layer and havinga working temperature which is lower than said predetermined workingtemperature, saidsecond layer totally sealing said first layer andthereby hermetically sealing said ferrite slug without appreciablesoaking of said second layer of glass into said ferrite slug; and a tiphaving a taper less than that of said tapered end portions over at leastone of said end portions of said slug for providing a gradual change ofindex of refraction for said electromagnetic waves as they traverse froman evacuated atmosphere to the mid-portions of said ferrite slug, saidtip being composed of a glass having a dielectric constant substantiallyequally intermediate that of said fer rite material and said evacuatedatmosphere.

References Cited in the file of this patent UNITED STATES PATENTS2,568,881 Albers-Schoenberg Sept. 25, '1951 2,745,069 Hewitt May 8, 19562,748,353 Hogan May 29, 1956

